Eighteen patients(19 affected hips and 17 unaffected hips) with Legg-Calve-Perthes' disease(LCP) were reviewed to evaluate the relationship between medial joint space widening and lateral subluxation of the femoral head. MRI was used to evaluate components in widened medial joint space in radiographs. There was increased cartilage thickness of the femoral head and acetabulum and increased joint tluid which represented synovial hypertrophy in the widened medial joint space in radiographs. Increased cartilage thickness was found at the mediai aspect of the femoral head and at the lateral and posterior walls of the acetabulum when compared to unaffected hips and normal control hips. Widening of the medial joint space was related to lateral subluxation of the femoral head during the fragmentation or remodeling stage, not always during the avascular stage, in LCP. Widening of the medial joint space did not change remarkably after a Salter osteotomy or femoral varus derotational osteotomy because of the remaining cartilage thickness.